1. Field of the Invention
The present invention relates to a control method for controlling displacement of a piezoelectric actuator, and more specifically to a control method capable of performing large displacements for a long time, and a position control device and disk device using this control method.
2. Background Art
When a piezoelectric element is used as an actuator, conventionally, a piezoelectric film can be electrochemically deformed by an applied voltage, its characteristic degrades, and further its function as a piezoelectric actuator can be damaged. This degradation is apt to occur especially when the voltage applied for increasing the displacement amount is increased. For suppressing the degradation, various control methods are proposed as the driving method of the piezoelectric actuator.
As the control method of the piezoelectric actuator, Japanese Patent Unexamined Publication No. 2002-288954 discloses a method of restricting the control voltage applied to the piezoelectric actuator to a threshold or below.
FIG. 7 is a block diagram illustrating the disclosed control method. Piezoelectric actuator 202 is connected to power supply 206 for driving the actuator, ammeter 204, and position control unit 208. Position control unit 208 controls the voltage of power supply 206 to provide a displacement amount set in piezoelectric actuator 202. Information on the current value measured by ammeter 204 is transmitted to position control unit 208. When the control voltage that is not lower than the threshold is applied to piezoelectric actuator 202, insulation resistance of the piezoelectric film forming piezoelectric actuator 202 further degrades.
In the control method discussed above, for suppressing such degradation of the insulation resistance, the applied control voltage is restricted to the threshold or below. When the control voltage exceeds the threshold, the time period of exceeding the threshold is restricted. The threshold is set as follows:                the threshold is set dependently on the material of the piezoelectric film; or        the characteristic between control voltage applied to the piezoelectric actuator and the current is measured, and the control voltage at which the current increases exponentially is set as the threshold.        
Japanese Patent Unexamined Publication No. 2003-141832 discloses a method in which a means for detecting the insulation resistance of the piezoelectric actuator during use is disposed, and, when the means detects the decrease in insulation resistance, the insulation resistance is recovered by applying high voltage.
FIG. 8 is a block diagram illustrating this control method. Piezoelectric actuator 210 has a three-layer structure where electrode films are formed on opposite sides of the piezoelectric film. Regarding the decrease in insulation resistance of piezoelectric actuator 210, generally, the resistance of the piezoelectric film does not wholly decrease, but part of the piezoelectric film often degrades locally. In such a case, high voltage is applied to the gap between the electrode films to make high current flow intensively to a part where the electric resistance is decreased by local degradation, and thus a breakdown phenomenon such as occurrence of peeling in the electrode film around the degraded part is caused. The breakdown phenomenon is used to remove the current path from a local low resistance region to recover the insulation resistance between the electrode films.
Specifically, as shown in FIG. 8, when a predetermined voltage is applied to piezoelectric actuator 210 from position control power supply 216, current flowing through piezoelectric actuator 210 is measured by ammeter 212, and current value information is fed into degradation determining unit 218. Degradation determining unit 218, based on the current value information, compares the fed current value with a preset current allowance. When this comparison indicates that the fed current value is higher than the allowance, a switch signal is fed into signal switching unit 220. By switching in signal switching unit 220, a set voltage is applied to piezoelectric actuator 210 from high voltage power supply 214. Thus, the current path can be removed from the local low resistance region as discussed above.
Another control method of a piezoelectric actuator is also shown in “IEEE (Institute of Electrical and Electronics Engineers) transactions on magnetics, vol. 35, No. 2 (1999) p. 984”. In this method, the voltage applied to the piezoelectric film is not set in a range of −X(V) to +X(V) (X is an arbitrary positive value) (typical voltage applying method), but is set in a range of 0 to X(V). In other words, the voltage range is restricted so that the voltage in the range 0 to −X(V) is not applied. In this case, the applied voltage is restricted to one-side polarity such as 0 to +X(V), and the applying direction is set to be the same direction as the initial polarization direction. As a result, the applied voltage is not applied in the direction opposite to the initial polarization direction. Therefore, the applied voltage does not cause inversion of the polarization, and the initial polarization is kept stable. When the initial polarization is kept stable, the displacement amount of the piezoelectric actuator is hardly reduced.
In the first control method of the piezoelectric actuator, long-term degradation of the actuator is suppressed by setting the absolute value of the control voltage at or below a certain threshold. The threshold is therefore set at a voltage value lower than the voltage value at which the piezoelectric actuator is not degraded by a short-term application. For example, the piezoelectric actuator is driven using, as the threshold, half the voltage value at which the degradation is not caused in a short time. The piezoelectric element hardly degrades, so that the lifetime is improved. The displaceable amount of the piezoelectric actuator is also restricted by the threshold voltage. Therefore, only a small displacement amount can be generated. In this conventional method, the displacement amount of the piezoelectric actuator and the lifetime are contradictory.
In the second control method of the piezoelectric actuator, the local breakdown phenomenon of the electrode film is used, so that the same desirable result cannot always be obtained. In other words, when high voltage is applied, the breakdown range of the degrading electrode film cannot be constant, namely the breakdown range can vary to be large or small. When a large area of the electrode film is broken, the area of the piezoelectric film to which the voltage is applied decreases greatly. Thus, the displacement amount of the piezoelectric actuator can greatly decrease, which is contrary to the objective of the method. The application of the high voltage can cause the piezoelectric element to dielectrically break down. When the application of the high voltage causes the electrode film to break down, dust is often generated. When the piezoelectric actuator is used for position control of the head slider of a hard disk device, for example, this generation of the dust can cause a failure of the entire hard disk device.
In the third control method of the piezoelectric actuator, the control voltage is applied completely in one direction, so that the movement of ions caused in the piezoelectric film by the control voltage is deflected in one direction. Accumulation of movement of ions disadvantageously causes the reduction of the insulation resistance.
The present invention addresses the conventional problems. The present invention provides a control method for controlling a piezoelectric actuator that can provide a large displacement amount and improve the lifetime of the piezoelectric actuator, and a position control device and disk device using the control method.